As is known in the art, an operational amplifier (or more simply “an op-amp”) is a DC-coupled high-gain electronic voltage amplifier having a differential input and typically having a single-ended output. An op-amp produces an output voltage that is larger (e.g. often hundreds or thousands of times larger) than a voltage difference presented at its input terminals. Op amps are important building blocks for a wide variety of electronic circuits. All op amps are compensated. Some are compensated with internal components thus saving a designer time and money. Many op amps are not compensated internally because leaving out the compensation gives the designer an extra degree of freedom. For proper use these op amps must have some kind of external compensation or they will oscillate. Furthermore, many internally compensated op amps are used in systems where additional compensation is necessary for proper operation.
One type of compensation is called “dominant pole” compensation. Another type of compensation is called “lag compensation”. A further type of compensation is called “lead compensation.” Lead compensation involves putting both a zero and then a pole into the loop transfer function. Conventional operational amplifier implementations rely on capacitors to provide the zeros required to implement all of these compensators. Many conventional systems are limited by internal capacitances which fundamentally limit the available compensation bandwidth. Furthermore, in conventional lead compensation schemes, adjusting the compensator fundamentally alters the closed loop behavior of the entire system.
FIG. 1 shows a non-inverting prior art compensation system that uses both inductors and capacitors to provide two lead compensators for the same op amp FIG. 2 is a block diagram associated with the circuit of FIG. 1. As can be seen from FIGS. 1 and 2, in a conventional implementation, a resistor R1 and inductor L1 are serially coupled to a negative input of an op amp 2 having a gain A and having a feedback network comprised of a resistor R2 and a capacitor C1. The feedback network provides compensation (and in particular, lead compensation) through frequency domain interaction of the resistors and capacitor and inductor.
From FIG. 2 it is apparent that changing values of the resistors R1 and R2, the capacitor C1, or the inductor L1 allows for compensation of the feedback loop through block 16. However, it should be noted that block 10 is the reciprocal of block 16, and therefore any changes made to compensate the circuit also change the closed loop transfer function of the system through its effect on block 10.
The explicit coupling of the compensator and the closed loop gain of the system complicates the design and limits the usefulness of the standard lead compensator implementation.
FIG. 3 illustrates a prior art compensation system for an inverting amplifier that uses both inductors and capacitors to provide two lead compensators for the same op amp.
FIG. 4 illustrates a prior art non-inverting compensation system that uses a resistor and capacitor shunt coupled between differential op amp inputs to provide lag compensation.